Deposition of coatings on substrates is performed using plasmas, such as thermal plasmas, expanding thermal plasmas (ETPs) and inductive coupling plasmas (ICPs). ETPs are produced by forming a thermal plasma under high pressure in an upstream portion of an ETP source and providing the thermal plasma to a downstream portion of the ETP source. The downstream portion includes a low pressure chamber (having lower pressure than the upstream portion) which receives the thermal plasma and a reagent precursor injection. Inside the low pressure chamber the thermal plasma expands due to the relatively large pressure differential between the downstream and upstream portions of the ETP source. Reagents that are injected into the expanding thermal plasma dissociate due to chemical reactions, such as charge exchange and subsequent dissociative recombination reactions, between the thermal plasma and the reagent.
Coatings deposited by individual ETP sources cover surfaces having a limited width typically less than 30 cm. Accordingly, multiple ETP sources are needed to coat large areas. The distribution of the energy level of the ETP is nonuniform, and is typically Gaussian shaped, resulting in a substantially Gaussian plasma density and a Gaussian deposition thickness profile. However, in most applications a uniform thickness profile for individual ETP sources is desired, particularly when multiple sources are used for coating a single surface.
Furthermore, in ETP sources the energy at which ions bombard electrically floating substrates, such as polymer substrates, is extremely low and uncontrollable. The tendency of ETPs to have low ion energy may interfere with generation of coatings having good adhesion and/or high density.
Attempts have been made to reduce pumping requirements and increase efficiency. These attempts include reducing the diameter orifice in a cascade plate of the ETP source for allowing ETP operation with a reduced plasma gas flow, which in turn reduces the ion flux that is needed for reagent dissociation and reduces deposition rate. Attempts have also been made to increase utilization by using a nozzle injector configured for intensive mixing of reagents and the expanding thermal plasma. However, the nozzle injector substantially confines the plasma, which tends to induce non-uniformities. Attempts have also been made to increase ion energy control by applying an independent bias voltage to the substrate to which deposits are being applied. However, this method does not function when processing electrically floating substrates, such as polymer substrates.
Nonetheless, ETPs have relatively high charged particle densities, relatively low electron temperatures, and maintain an equilibrium between electron temperatures and ion temperatures, resulting in relatively low ion temperatures and bombardment energies, which is desirable for semiconductor applications and the prevention of damage to electronic devices. Although the low electron temperature of ETPs is considered to be advantageous, there exist nonobvious advantages for raising the electron temperature. While ETPs provide a reagent dissociation path based on charge exchange reactions followed by dissociative recombination reactions, which occurs at relatively low electron temperatures, the ETP's Te is too low for providing a reagent dissociation path based on electron impact dissociation reactions.
Each path provides the ability for generation of specific chemical species, and the lack of the electron impact dissociation path limits the type of chemical species that can be generated, which limits contribution to the coating process and decreases efficiency of individual ETP sources. Thus, causing further demands are placed on the pumping process, including increases in gas and energy loads, the number of ETP sources required for large area uniformity and costs.
ICPs, on the other hand, are formed by ICP sources that include a low pressure chamber that receives a reagent precursor injection in a low pressure chamber, where the chamber is provided with at least one coil connected to an energy source. When energy is applied the at least one coil the Te increases and an electron impact dissociation path is achieved. However, ICPs, similar to ETPs, typically do not provide a uniform deposition over large areas. Further, in ICPs charged particle densities are typically low relative to ETPs, which limits the density of reagent fragments formed through dissociation reactions of reagents with energetic electrons, and thus results in low growth rates of coatings.
Higher power ICPs have been attempted for generating higher charged particle densities, but high power stray capacitive power coupling typically develops, causing sputtering of reactor walls, and thus contaminating the coatings that are deposited. Also, as a result of the ICP plasma generation mechanism the electron temperatures are not generally lowered below a certain energy, thus limiting the plasma chemistry that can be achieved with the ICP source.
Accordingly, there is a need for a system and method for uniformly distributing dissociated reagents over the surface of a substrate, thus forming a coating on a substrate.
Furthermore, there is a need for a system and method for efficiently generating a high electron density plasma with a controllable electron temperature, such that ICP like high Tes can be combined with ETP like electron densities, resulting in high growth rates with controllable plasma chemistry based on both charge exchange and dissociative recombination paths and electron impact dissociation paths. Other advantages are described in greater detail below.